This invention relates to metal discrimination detection devices.
It is now common art for the remote detection of metal to generate and transmit through a first coil a signal and to detect the magnetic effects of that signal. Conventionally the signal is effected by an alternating current signal which is fed to the transmission coil. Detection electronics then compare a received signal induced in a receiver coil with the transmitted signal.
The induced E.M.F. in the receiver coil will carry information by reason of its phase delay and magnitude, and it is conventional that any such signal is referred to by two quadrature components one of which is the zero phase delay component relative to the transmitted magnetic field (or the "resistive" component) and the other is the quadrature or 90.degree. phase shift component which shall be referred to as the "reactive" component. When a metal detector of this type is being used in conventional circumstances, the head which is conventionally separate from the detection electronics is passed over the ground, and some attempt is usually made to adjust the detection electronics so that the ratio of reactive to resistive components contributed by the ground is used as a zero reference so that any subsequent change in this ratio may be detected, thereby detecting possibly useful target has been detected.
Since the ratio of resistive to reactive components will vary from location to location with different ground types, it is necessary that there be some means by which a detector can be brought to a condition whereby the ground is considered as a zero reference. This is usually achieved by some manually adjustable device such as a potentiometer which controls in the electronic components a phase reference device. Obviously, once the metal detector has been set for that ground to be a zero reference, moving the detector coil across ground in the immediate vicinity would be expected to provide the best sensitivity for detection of appropriate targets which have resistive to reactive ratios different to that of the ground.
The problem to which this invention is directed is that it has been discovered with metal detectors of previous design that ground balancing which is important to achieve best sensitivity especially in soils which have a proportion of magnetic material such as magnetite depends upon not only characteristics of the soil but also factors relating to the relative positioning of the head relative to that soil. In practise with current available detectors optimum ground balancing has been discovered to be a function of the distance between the ground and the sensing head. However, a metal detector will be used over ground which will vary in makeup, and may have randomly located protuberances. Further, the equipment will be carried by persons at a varying heights. This essentially means that if ground balancing is to be kept at an optimum the head should be kept at a uniform height about the ground especially in magnetic soils. As in practice this is not possible in most circumstances, the sensitivity and efficiency in being able to effectively detect an anomoly is severely reduced.
The problem has been to establish just why conventional so called ground balancing cannot yield a response which is independent of the distance between the ground and sensing head in many conditions that might be encountered in practice, especially in the so called magnetic soils. Since magnetic soils are often the host of the valuable target metals such as gold that are being sought, this is a serious deficiency and it is an object of this invention to propose means by which there can be more effective detection and maintenance of ground balancing especially in magnetic soil conditions.